Page 262 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 11. Integration and outlook
concentrations of Fe(II) and Fe(III) of >30 and >10 mg L-1, respectively. Where iron cycling was often linked to suppression of methanogenesis, and as alternative electron acceptor in organic matter mineralization, its potential role in AOM and methanogenesis should be further investigated (Lovley and Phillips 1986; Van Bodegom, Scholten and Stams 2004; Miller et al. 2015). Even more so, recent studies indicated that Methanosarcina species might use iron oxides to boost their acetate metabolism or even use it to oxidize CH4 (Holmes et al. 2019; Ferry 2020). See also Marquart et al. (2019) on the relation between iron reduction and methanogenesis.
Anaerobic metabolisms by aerobic methanotrophs
Aerobic methanotrophs often provide the most important and significant CH4 filter (Chowdhury and Dick 2013). Their relevance is partly explained by the high activation energy needed to oxidize CH4 (Tang et al. 2014), but best by the higher energy yields of aerobic CH4 oxidation (ΔG0´ = -801 kJ mol CH4-1) vs. anaerobic CH4 oxidation (ΔG0´ = -21 to -503 kJ mol CH4-1). The niche separation between aerobic methanotrophs, and anaerobic methanotrophs and methanogens, is generally well explained by gradients of oxygen and alternative electron acceptors. However, recent evidence suggests that aerobic methanotrophs are not necessarily strictly associated to oxic environments. The recent discovery of anaerobic strategies within aerobic methanotrophic bacteria had a considerable impact. This finding is, however, not entirely novel. The potential for nitrate reduction is found in both alpha- and gammaproteobacterial methanotrophs (Stein and Klotz 2011; Kits, Klotz and Stein 2015). In gammaproteobacterial methanotrophs, nitrate reduction is coupled to CH4 oxidation to conserve energy under hypoxia (Kits, Klotz and Stein 2015). Dissimilatory nitrate reductases are present in alphaproteobacterial methanotrophs (Shapleigh 2011; Stein and Klotz 2011). The link with metal reduction and fermentation pathways is, however, novel (Gilman et al. 2017; van Grinsven et al. 2020; Zheng et al. 2020). How these aerobic methanotrophs are able to perform iron-AOM under hypoxia and the absence of known iron cycling genes in their genome remains elusive (Bar-Or et al. 2017; Cabrol et al. 2020; Zheng et al. 2020).
A study on anaerobic oxidation of methane in anoxic water of Northwestern Siberian lakes found Methylomonadaceae (Methylobacter and Crenothrix) as potential major methane- oxidizing bacteria (MOB) in the anoxic water columns (Cabrol et al. 2020). Interestingly, they
260